Question: The arithmetic sequence $(a_i)$ is defined by the formula: $a_i = -9 + 6(i - 1)$ What is $a_{4}$, the fourth term in the sequence?
Solution: From the given formula, we can see that the first term of the sequence is $-9$ and the common difference is $6$ To find $a_{4}$ , we can simply substitute $i = 4$ into the given formula. Therefore, the fourth term is equal to $a_{4} = -9 + 6 (4 - 1) = 9$.